It is well known that acrylic fibrous materials when subjected to heat undergo an exothermic thermal stabilization reaction wherein the fibrous material is transformed to a black form which is non-burning when subjected to an ordinary match flame.
Such modification generally has been accomplished by heating the acrylic fibrous material in an oxygen-containing atmosphere. It is believed that the resulting thermal stabilization reaction involves (1) an oxidative cross-linking reaction of adjoining molecules as well as (2) a cyclization reaction of pendant nitrile groups to a condensed dihydropyridine structure. The cyclization reaction is exothermic in nature and must be controlled if the fibrous configuration of the acrylic polymer undergoing stabilization is to be preserved. The resulting thermally stabilized fibrous material is useful as a precursor in the formation of carbon fibers.
On a commercial scale the thermal stabilization reaction commonly is carried out on a continuous basis with a continuous length of a multifilament acrylic fibrous material being passed in the direction of its length through a thermal stabilization zone which is provided with a heated gaseous atmosphere. The movement of the continuous length of acrylic fibrous material through the stabilization zone may be directed by rollers, etc. situated therein.
When the continuous lengths of acrylic fibrous material is in the form of a flat band (e.g., a flat tow), a need has arisen for an improved flat splice which can be used to effectively join the segments of the precursor so that the continuous operation of the thermal stabilization reaction will not have to shut down after the passage of each length of fibrous precursor through the furnace. For instance, acrylic tows are commercially available in the form of a bale having a finite filament length. Considerable down time commonly is involved to cool down the thermal stabilization zone, to string up another length of fibrous precursor, and to again bring the thermal stabilization zone to operating temperature. Such restringing also wastes a portion of the precursor fiber. Accordingly, those skilled in the art have been aware of the need for an effective splice technique whereby the thermal stabilization treatment may be conducted for an extended period of trouble free operation.
It has been found that the exothermic heat of reaction commonly creates difficulties in connection with prior splice attempts since resulting heat is not effectively dissipated and may cause breakage of the fibrous band. Also bulky splices (e.g., knots or splices which attempt to utilize non-flexible adhesives) commonly cannot be accommodated within thermal stabilization zones without interference and loss of process stability. If the band segments are laterally spread prior to conventional splicing in an overlapping fashion in an attempt to deal with the usual exothermic heat of reaction, then the substantially wider section of the band at the splice area complicates processing as the band travels on a continuous basis.
Further, problems arise when attempting to provide a splice due to the high temperatures encountered in the thermal stabilization zone as well as the additional heat generated by the exothermic reaction. One method of overcoming the above described problems is found in U.S. application Ser. No. 641,066, filed Dec. 15, 1975 wherein the end of each band segment is surrounded with and attached to a fabric which is capable of withstanding the heated thermal stabilization. A portion of said fabric extends beyond each enclosed end which extension is sewn together with a similar extension provided on a second band segment. Such a process, however, includes two sewing steps which are time consuming. The search has continued therefore for a way to provide an improved splice for forming the above described band segments which possesses the dual requirements of stability under the elevated temperatures of thermal stabilization, and the ability to dissipate heat to an extent sufficient to avoid the deleterious effects on the original fibrous configuration of the exothermic reaction which might result in the absence of said dissipation.
It is an object of the present invention to provide an improved process for the thermal stabilization of a substantially flat multifilament band of an acrylic fibrous material where the ends of at least two discrete band segments are joined and continuously are passed in a successive manner in the direction of their length through a heated thermal stabilization zone while maintaining the original fibrous configuration of the same substantially intact for a residence time sufficient to render the band black in appearance, and non-burning when subjected to an ordinary match flame.
It is an object of the present invention to provide a substantially flat multifilament band of an acrylic fibrous material which is capable of undergoing thermal stabilization by continuous passage through a thermal stabilization zone in the direction of its length having at least two discrete band segments which are joined in an end relationship by an improved splice.
It is an object of the present invention to provide a flexible splice for joining in an end to end relationship at least two discrete band segments of an acrylic fibrous material wherein the exothermic heat of reaction resulting from a thermal stabilization reaction is readily dissipated through the splice to an extent sufficient to avoid the deleterious effect on the original fibrous configuration which might otherwise result in the absence of such dissipation.
It is a further object of the present invention to provide a flexible splice for joining in an end to end relationship at least two discrete band segments of an acrylic fibrous material which is relatively flat in configuration and readily may be passed through a thermal stabilization zone without impairment of movement resulting from the presence of the splice.
These and other objects as well as the scope, nature, and utilization of the invention will be apparent from the following description and appended claims.